A significant impediment to the increased use of ceramic materials in certain applications is the high incidence of failures in engineered ceramic parts. These failures can often be attributed to small cracks or voids in such parts, which result from incomplete packing of the precursor powders. One solution to this problem is the manufacture of monodispersed powders which can be packed tightly, thereby reducing the void spaces between particles.
Current efforts in ceramic technology are directed toward the manufacture of ceramic parts that exhibit the desirable physical properties of the material, e.g., hardness, maintenance of structural integrity at high temperatures, and chemical inertness, with the elimination of impurities and defects which often result in failure of the ceramic. It has been suggested, by E. A. Barringer and H. K. Bowen, in "Formation, Packing and Sintering of Monodispersed TiO.sub.2 Powders", J. Amer. Ceram. Soc. 65, C-199 (1982), that an "ideal" ceramic powder for producing a high quality part must be of high purity and contain particles which are monodispersed, spherical, nonagglomerated and of a particle size 0.1-1.0 micron in diameter.
As a ceramic powder is sintered, adjacent particles fuse into grains. In general, the grain size is governed by the crystallite size within the particles from which the part is prepared. In other words, the grain size is necessarily larger than the crystallites from which a part is sintered. Thus, the sintering of finer particles presents the opportunity to produce fine-grained bodies.
An additional advantage in the use of ceramic powders with a fine uniform particle size is that the temperatures required to sinter the powders are often reduced. In one work describing sintering TiO.sub.2 powders, two researchers, Barringer and Bowen, found that the sintering temperature could be reduced from 1300.degree.-1400.degree. C. to 800.degree. C. when using 0.08 micron-sized particles. On an industrial scale, this could result in a considerable savings both in material and energy costs.
Titanium diboride powder (TiB.sub.2) may be prepared by a number of methods including the reaction of elemental and crystalline titanium and boron compounds at high temperatures (2000.degree. C.), the reduction of the oxides, the reaction of a titanium source with boron carbide, or the vapor-phase reaction of titanium halides with boron halides (chlorides and bromides) in a hydrogen plasma. In the latter process, the endothermic reaction is driven by heating the reactants to a temperature significantly above the spontaneous reaction temperature in a hydrogen plasma to form submicron titanium diboride particles. The major fraction of particles comprising the powder product have a particle size in the range between 0.05 and 0.7 micron. The resultant titanium diboride powder can be hot pressed or cold pressed and sintered to articles having densities of at least 90, e.g., 95 percent of theoretical. U.S. Pat. No. 4,282,195 describes one such process for preparing submicron titanium boride powder from titanium tetrachloride and boron trichloride in a vortex-stabilizing hydrogen plasma. These plasma produced powders consist of a mixture of both submicrons and micron-sized particles. In most cases, the powders contain a substantial fraction of particles (as much as 10 percent) with diameters greater than one micron. In addition, the powders contain a large amount (4000 ppm) of metal impurities, introduced by the plasma apparatus itself.
The synthesis of ceramic powders using a carbon dioxide laser was first developed by Haggerty and coworkers. In their article, "Synthesis and Characteristics of Ceramic Powders Made from Laser-Heated Gases", Ceram. Eng. Sci. Proc. 3, 31 (1982), wherein R. A. Marra and J. S. Haggerty describe the preparation of silicon, silicon carbide and silicon nitride powder by driving exothermic reactions involving SiH.sub.4. The ultrafine powders produced are equiaxed, and mono-dispersed with particle sizes in the range of 0.01-0.1 micron. Marra and Haggerty further state that this laser-heated process can be used to produce other and nonoxide ceramics such as TiB.sub.2, aluminum nitride (AlN), boron carbide (B.sub.4 C), as well as many oxide ceramics. See: Sinterable Ceramic Powders From Laser-Driven Reactions, Process Description and Modeling," W. R. Cannon, S. C. Danforth, J. H. Flint, J. S. Haggerty, and R. A. Marra, J. Amer. Ceram. Soc. 65, 324 (1982), J. Amer. Ceram. Soc. 65, 330 (1982); "Synthesis and Characteristics of Ceramic Powders made from Laser-Heated Gases," R. A. Marra and J. S. Haggerty, Ceram. Eng. Sci. Proc., 3, 31 (1982); "Apparatus for Making Ultrafine Particles", Jpn. Kokai Tokkyo Koho. JP 56-13664 A2 [81-136664] 26, Oct., 1981; and "Submicron Titanium Boride Powder", U.S. Pat. No. 4,282,195 (1981).